RF tags are in widespread usage in such areas as tracking of locations of persons, e.g., doctors in hospitals, parties entering and leaving secured areas, and articles, e.g., as in the case of various known electronic article surveillance systems.
In the person tracking applications, persons wear RF-interrogatable tags which have memory for storing a "preamble", i.e., an introductory message code portion common to the particular person tracking system, and an ensuing message code portion uniquely indicating an identifying coding specific to the person bearing the tag. The tags have further facility for detecting an interrogating RF carrier in which the preamble message code portion is present and respond thereupon with a transmission containing the tag memory content, i.e., the preamble and the person-identifying coding.
In the article surveillance situation; articles are equipped with tags which respond to certain interrogations, e.g., at store or library exits, to cause RF receivers at such locations to indicate that the article tag has not been deactivated. Tag deactivation customarily occurs at a checkout station, but is sometimes overlooked or ineffectively implemented, such that the tag remains armed to respond to interrogation. The article surveillance tags may be of a type simply resonating in response to incident RF energy, and providing RF detectable energy bearing identifying modulation content, based on a nonlinear element, e.g., a diode in the tag resonant circuit which reacts differently to positive and negative cycles of the system carrier. In article surveillance systems, the responses of all participating tags provide generally identical outputs and do not afford individual article identification as in the case of the person-tracking systems.